Conventionally, a hierarchical storage system which controls a file storage destination based on access frequency has been proposed and put to practical use (see PTL1). In a hierarchical storage system of this type, a plurality of storage apparatuses of different response performance and reliability are provided and the storage apparatus with the highest response performance and reliability (which will be called the primary storage apparatus hereinbelow) is provided to a client/host. Furthermore, in a hierarchical storage system, the access frequency of files which the client/host stores in the primary storage apparatus is monitored and files with a low access frequency are migrated to another storage apparatus such as an archive apparatus (which is called the secondary storage apparatus hereinbelow).
In this kind of hierarchical storage system with a built-in data management function, if a file which is stored in the primary storage apparatus is migrated to the secondary storage apparatus, a file with meta information containing address information on the file migration destination (hereinafter called a stub file) is stored at the source storage location. Further, if a read request to read the file migrated to the secondary storage apparatus is issued, the primary storage apparatus acquires the corresponding file from the secondary storage apparatus on the basis of the address information contained in the stub file and, as a result of transferring the acquired file to the read request transmission source, handles the file as if the file were at the source storage location. This type of handling is known as recall.
Furthermore, conventionally, a technology in which a snapshot is applied to the foregoing hierarchical storage system has been proposed in PTL2. Here, a snapshot refers to a static image of a snapshot acquisition target at a certain point in time.